Just to confirm- the username of an existing Spotify account cannot be changed.
If you create a new account, a username consisting of numbers and letters will be created automatically. This is happening out of security reasons.
You will also have a display name. This is what all users will see when they come across your account. You can choose a display name in the 'What should we call you?' field during sign up.
If you wish to have a different username, you must create a new account and you can transfer all of your music to it. You can read more about it here.
Don't forget to cancel your subscription on your previous account and subscribe to Premium on the new account. If you have subscribed to Spotify through a partner of ours, you must reach out to them so they can fix this for you.
We hope this sheds light onto the subject.
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